Perception of illumination and shadows Lavanya Sharan February 14th, 2011.
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Transcript of Perception of illumination and shadows Lavanya Sharan February 14th, 2011.
Studied indirectly
•Not a lot of studies examine illumination or shading directly
•Role of illumination and shading in perception of 3-D shape, reflectance, object identity and space
Shape from shading is under-constrained.
Fig 9.11, VPfaCGP
And yet, we perceive unique and stable shapes.
Theoretical cues for shape from shading
• Reflectance map (Horn, 1977)
• Isophotes (Koenderink & Van Doorn, 1980)
• Image orientation (Fleming, Torralba & Adelson, 2004)
Theoretical cues for shape from shading
Reflectance map (Horn, 1977)
•Representation of scene brightness as a function of 3-D surface orientation
•Ignores shadows, inter-reflections, vignetting, translucency etc.
•Unclear whether this relationship between image intensity & surface orientation is used by visual system
Horn & Sjoberg, 1978
Theoretical cues for shape from shading
Isophotes (Koenderink & Van Doorn, 1980)
•Curves of constant intensity, depend on illumination and shape
•Patterns of isophotes can reveal shape (under assumptions of lighting)
•The visual system could use these as a cue
Fig 9.12, VPfaCGP
Theoretical cues for shape from shading
Image orientation (Fleming, Torralba & Adelson, 2004)
•Orientation filters have strong responses for strong curvature regions.
•By measuring these across a surface can get local geometry
•The visual system could use this relationship between image orientation and surface curvature. Fig 9.13, VPfaCGP
Why is shape from shading hard?
Lots of ambiguities.
• Convex vs. concave?
• Surface orientation change vs. surface reflectance change?
• Bas-relief ambiguity
What have we learnt from gauge figure tasks?• Subjects are consistent. Their (inferred)
shapes are related by affine transforms. (Koenderink et al., 1992)
• For simple shapes, contours are often enough for estimating shape, shading plays a lesser role. (Mamassian & Kersten., 1996; Koenderink et al., 1996; Cole et al., 2009)
• Illumination changes causes subtle distortions of perceived shape. (Koenderink et al., 1996; Caniard & Fleming, 2007)
Intrinsic image analysis
Fig 9.15, VPfaCGP
Idea: Visual system separates retinal image into layers that represent distinct physical causes. (Barrow & Tenenbaum, 1978)
How? Proposals include Retinex, anchoring theory, etc.
Mutual illumination affects reflectance perception
Ruppertsberg & Bloj, 2007
We can distinguish black and white rooms seen in isolation based on inter-reflections. (Gilchrist & Jacobsen, 1984)
Mutual illumination estimation is not perfect, sometimes perceived as surface color. (Bloj et al., 1999; Doerschner et al., 2004)